Wisconsin's Role in Great Lakes Restoration - American Water ...

20. Hydrodynamic Drag of Native Wisconsin Wetland Plant Species
Zachariah P. Zopp, University of Wisconsin-Madison, Madison, WI, zopp@wisc.edu
Anita M. Thompson, University of Wisconsin-Madison, Madison, WI, amthompson2@wisc.edu
Aquatic macrophytes and terrestrial plants generate drag in the presence of flowing water,
thereby reducing water velocity and enhancing particle deposition. The use of native
Wisconsin wetland plants to treat urban stormwater is dependent, in part, on their ability to
generate drag and reduce flow velocities. To date, this topic has remained largely
unexamined. For this study, drag forces on ten native Wisconsin wetland plants, categorized
by three groups (forb, graminoid, and grass) were measured in a laboratory flume. The flume
simulated expected flow velocities and depths through wetland swales established at the
University of Wisconsin-Madison Arboretum to treat urban runoff.
Experimental treatments consisted of three plant growth periods (two, four, and six months),
four flow velocities (0.08, 0.15, 0.23, 0.30 m/sec) and three flow depths (0.06, 0.18, 0.30 m).
The plants were grown in a greenhouse with similar lighting and temperature conditions to
that of a Wisconsin summer. A representative shoot from each species was attached at its
base to a 0.32 cm diameter vertical steel rod. The steel rod connected to a horizontal
aluminum air sled above the flume. The sled hovered on a 360° stream of compressed air,
which allowed the sled to move without friction. The sled was connected to a 1.5 Newton load
cell that measured the drag force generated by the plant.
A plant’s hydrodynamic drag depends on several factors such as foliage, flexibility, and shoot
diameter and density. In addition to drag force, plant wetted frontal area and shoot bending
angle were measured for each plant and flow condition. Relationships between drag forces as
well as dimensionless drag coefficients and Reynolds numbers will be examined and
presented. Results will be used to identify native Wisconsin wetland species and/or classes
that are best suited to reduce stormwater runoff velocity in treatment systems. This study is
one component of a collaborative project conducted by the University of Wisconsin-Madison,
the Wisconsin Chapter of the Nature Conservancy, and the Environmental Law Institute to
investigate innovative approaches to stormwater treatment. Information gained from this study
will guide watershed based planning for wetland restoration and mitigation in the Pensaukee
Basin of Wisconsin.
46